In technical applications, such as in an automobile construction and the like, for example, it is desirable to quickly and reliably detect the location of a component that can be moved into two end positions relative to a stationary part, using measurement technology. In retaining systems of automobiles, for example, it is desirable to check, for example, whether or not a passenger is belted. For this purpose, proper locking of the tongue of the seat belt that has been inserted into the belt lock is checked. Knowledge of the locking status of the belt lock allows for the passengers to be notified by a signal to put on and fasten their seat belts. Since the introduction of safety airbags, information about the locking status of seat belt systems has also been important for activating or deactivating mechanisms for inflating driver and passenger airbags or side and head airbags.
For example, Hall sensors are known for contactless monitoring of components that change their location and especially can occupy two different end positions. Hall sensors can include a semiconductor layer supplied with a constant current, in an integrated design. A magnetic field component perpendicular to the semiconductor layer influences the constant current, and the sensor delivers a Hall voltage that can be evaluated, that can be tapped and used to evaluate a status, or that can also be used directly as a switching voltage. The integrated design of Hall sensors makes it possible to integrate an evaluation circuit that is suitable for evaluation of the switching state on the Hall sensor. In the automotive industry, therefore, Hall sensors are used as contactless status sensors in many applications.
EP-A-0 861 763 discloses, for example, a belt lock with an integrated, pretensioned Hall sensor that, without contact, detects the state of a locking body or an ejector for a lock tongue that has been inserted into the belt lock. A Hall sensor with a Hall field is arranged in direct proximity to a permanent magnet. Changing the location of the locking body and of the ejector, which are composed of a ferromagnetic material for this purpose, changes the magnetic field of the permanent magnet. In doing so, the signal of the Hall sensor changes and at the output of the Hall sensor, the status change can be tapped as a voltage change. In one alternative variant, it is suggested that the Hall sensor with a Hall field can be installed without a permanent magnet, and the locking body or the ejector can be designed as a permanent magnet for this purpose. In this arrangement, the change in the location of the locking body or of the ejector will also be detectable by a change of the Hall voltage.
However, in the belt lock according to EP-A-0 861 763, the Hall sensor must be positioned very carefully with reference to the locking element or the ejector. Subsequent installation of the Hall sensor can therefore be relatively complex and expensive. Depending on its arrangement, the Hall sensor can also be sensitive to external stray electromagnetic fields that can be caused by, for example, a magnetic key ring. Optionally, even additional shielding may have to be mounted, which complicates the structure or installation. The susceptibility to external stray fields is also increased by the signal changes being relatively small due to the relatively short paths that must be traversed by the locking body or the ejector when the seat belt lock is locked or unlocked. The seat belt variant without a pretensioned Hall sensor, in which either the locking body or the ejector is designed as a permanent magnet, is less practicable. The attainable signal changes are also relatively small here, which makes it difficult to detect different states, such as whether the belt lock is locked or unlocked. Vibrations of the locking body and of the ejector during locking and unlocking of the seat belt can cause demagnetization of the permanent magnet with time. This can ultimately lead to the Hall sensor becoming ineffective and the status changes of the belt lock no longer being able to be reliably detected.
The known belt locks have a very compact design that therefore limits the available space within the belt lock. This makes it difficult to arrange sensor components within the belt lock housing, especially in the vicinity of a component that changes its location from one end position into the other end position when the belt lock is actuated. If shields are then also to be attached, the engineer is generally faced with an essentially insoluble problem since the dimensions of the belt lock housing are not to be changed.
EP-B-1 485 276 discloses a belt lock in which the locking status is by a switch that can be actuated mechanically. The switch includes a fixed contact sheet and a contact sheet that is designed as a spring contact and that projects into the displacement path of a slide that can be moved into two end positions. During locking, the slide presses against a middle bent region of the spring contact, by which a hammer-shaped contact end comes into contact with the fixed contact sheet. However, this known belt lock switch is susceptible to failures. For example, if the spring contact is made too solid, it can disturb the displacement motion of the slide, and by blocking the slide in a middle position, it can even lead to malfunction of the belt lock. If, conversely, the spring contact is made so thin that its inherent spring force cannot impede the slide in any case, it tends to rattle during operation of the automobile. There is also a certain danger that the middle bent region of the spring contact will deform with time. This can lead to the hammer-shaped contact end no longer coming into contact with the fixed contact sheet and in this way the locking status of the seat belt is no longer detected and/or indicated. In the worst case, the spring contact even breaks due to continuous vibrations, which can likewise lead to the locking status of the belt lock no longer being able to be detected.